Moore's Law savior EUV faces uncertain future

CPTF 2012 The optical lithography that etches the chips in your digital devices is reaching its limits, but exactly when its oft-touted replacement – extreme ultraviolet lithography, commonly known as EUV – will be ready for prime time remains unclear.

"There are still some technical challenges which, of course, lead to a certain degree of uncertainty as to when, exactly, EUV will become available," said IBM Distinguished Engineer Lars Liebmann, speaking at the Common Platform Technology Forum 2012 on Wednesday in Santa Clara, California.

Liebmann should know. As part of IBM's semiconductor R&D team, he focuses on research into "design technology and co-optimization for sub-resolution patterning of leading-edge technology nodes" – meaning that he's figuring out how to etch chips with smaller and smaller features.

Currently, "leading-edge technology nodes" are etched with the optical immersion-lithography technology known as 193i, which TSMC and IBM started using at 45nm, and Intel began using at 32nm. Unfortunately, as chip process sizes are shrinking to 14 nanometer and beyond, 193i is reaching the end of its usability.

Tellingly, Liebmann's talk was entitled "The End of Optical Lithography", and its core focus was on how the transition would be made from 193i to EUV – and on the challenges of getting from here to there.

EUV is a radically different lithography technology from 193i optical, Liebmann explained. Not only is its wavelength significantly shorter at 13.5-14nm compared with optical's 193nm, but the light is derived not from the argon fluoride (ArF) excimer laser used for 193i but, instead, from a plasma light source.

In addition, the EUV operation happens in an extreme vacuum, and not in ambient atmosphere, and instead of photons being the etching agents, Liebmann said, "you're actually relying on secondary electrons to trigger the reactions."

Liebmann also explained a number of other differences – such as the use of reflective masks and a completely different etching chemistry, but his core message was that the move to EUV is not an evolutionary step as was the move to 193i, but instead a revolutionary change.

"I just mention that," he said, "because early on when some brilliant mind renamed projection x-ray lithography to EUV lithography, people got this impression that, 'Oh, deep EUV? It's pretty much the same thing.' No, it's a fundamentally different approach."

Significant challenges remain to be sorted out, he emphasized. For one, at its current state of development, EUV is currently "at least one, maybe two orders of magnitude too low on the intensity," he said.

Some of EUV's complexities are due to how its light is generated. "Exploding microdroplets of tin in a vacuum with a high-powered laser to make light is a very complicated process," Liebmann said, in quite the understatement.

Part of the problem in EUV's research and development process, he said, is that "until you get sufficient flux out of your lightbulb, it's very difficult to develop the chemistry" of the resists being etched.

Diving deeper into process technology geekery, he explained this problem, saying, "If you improve the sensitivity of the resist to make up for the low source power, now you get into shot noise, and you end up with very rough sidewalls."

What's an EUV boffin to do, eh?

And if EUV fails entirely..?

Defects are also extremely difficult to detect in an EUV system, a situation that's not helped by the fact, Liebmann said, that "the industry as a whole has given up on making defect-free blanks. The plan is to make reasonably defect-free blanks, and then pick a blank to match the chip design, and then align the chip design around the remaining defects so that the defect won't cause any harm."

All these challenges – and more – have been under investigation for quite some time. "We have people within the alliance that have been spending their entire careers on EUV lithography," Liebmann said. "We have some members on the team who have been working on this for literally three decades."

When these researchers explain the problems to him, Liebmann said, "I give them a lot of credibility that when they say there are still some problems remaining, that they actually know what they're talking about."

That said, the plan is now to have EUV up and running at a level that could support high-volume integrated wafer development in 2013, and full-scale deployment at the 14-nanometer process node in or around 2015. "It's going to be extremely challenging to meet that," Liebmann said. "I would say that it's going to be impossible to pull that in from that date."

Liebmann gave a nod to work that is being done on one EUV alternative, maskless massively parallel electronic beam lithography (EBL), citing work being done by Advantest, KLA-Tencor, Vistec, and Mapper. The many varients of EBL essentially eliminate the mask, and use tens or hundreds of thousands of electron beams to do the etching.

"E-beam lithography itself is pretty straightforward," Liebmann said. "We all sort of qualitatively understand how this all works. And we all also understand that the biggest problem is throughput. What all of these systems are working on is massively parallelizing the system to get to a point where you can begin to make this profitable."

From Liebmann's point of view, however, EBL is still not up to snuff. While he's impressed with the companies' prototypes, "None of them are really mature enough at this point even for one of the big exposure-tool companies to pick a solution and run with it."

According to Liebmann, the industry is waiting to see which of the competing EBL technologies is going to be picked up. "Where's Nikon going to go with all of this?" he asked. "Are they going to bet on one of these solutions?"

That said, he and his fellow researchers are keeping their eye on EBL. "We feel that this will be an absolutely vital technology if EUV continues to slip, or if EUV runs into some catastrophic issues, and we have to do the 10 nanometer technology node without EUV."

To stay on schedule for 14nm, Liebmann said, his research group has come up with a transition scheme that doesn't rely on EUV. "We have to do the technology development, we have to do the technology ramp, we may even have to do some early customer demos without relying on EUV, so we'll keep moving forward with 193. To do that, we have to enable some fairly aggressive double patterning or multiple patterning solutions."

To move from double- or triple-mask patterning using 193i to EUV, the 14nm chip layouts will have to be migratable from the optical solutions to the eventual EUV solutions without too much "churn", as Liebmann put it, on the design side. To accomplish that, chip designers will have to keep both optical and EU considerations in mind.

But ultimately, he said, everything should transfer from optical 193i into EUV at the 14nm node. "This is going to be the worst of nodes and the best of nodes, all in one node," he said. "As engineers, it's not going to get boring anytime soon."

After his presentation, Liebmann was asked when he thought EUV would finally be stable, reliable, and have enough throughput for mass production. "Not before 2015," he said. "I shouldn't paint too pessimistic a picture, but you've seen that there's some very severe technical challenges."

The future of EUV is still not certain, he emphasized. "I worked on proximity x-ray lithography for many years," he said. "EUV is still not at the level of maturity where x-ray lithography was when we found out that it's not going to work." ®